Attempts are continuously being made to increase the efficiency of the propulsion of watergoing vessels. Hull and propeller design must be interdependently adjusted to reduce cavitation and the production of vibration. It is also necessary to make the water flow back through the propeller as uniform as possible so the rotation of the propeller is transformed efficiently into backwardly directed thrust. In general any decrease in cavitation, vibration, and turbulence results in increased propeller efficiency.
In particular "Vibration Problem With a Class of Cargo Liner and the Solution From Fitting a Fin" (K. Takekuma, Transactions of the Royal Institution of Naval Architects, paper 18, 1979) describes how a stern tunnel fin is fitted to a ship to reduce the drawing of bubbles down into the propeller vortex. Such a stern fin projects horizontally from both sides of the hull from locations immediately ahead of and above the propeller. Each such fin is of airfoil section and has a thin leading edge that is somewhat below its thicker trailing edge, relative of course to the normal forward direction of travel of the ship. The purpose of the fin is to deflect upward the bubbles drawn underwater by the ship's bow wake. Such a solution relates to a particular vibrational problem, and is indicative of the intensity of work in this field.
Nonetheless it is still true that the efficiency of a ship propulsion system can be improved upon.